The present invention relates generally to balancing of rotors and, more specifically, to low-speed balancing of a rotor for a gas turbine engine, for example, for obtaining improved high-speed balance of the rotor.
A low-speed rotating machine is defined as a system having a rotor which operates well below its first flexural critical speed (i.e. sub-critically). It is well known in the art that such rotors may then be balanced before operation in conventional low-speed or two-plane balance machines. Since their rotors never run fast enough to experience any major vibratory flexure due to resonance, the low-speed balance procedure is also referred to as rigid-body balancing.
A conventional low-speed balancing procedure involves supporting the rotor on two soft-mounted slave bearings in a low-speed balance machine, each bearing being equipped with a displacement transducer which can detect the motion at that bearing induced by a rotating unbalance. The machine may then be calibrated by imposing known unbalances on the rotor. Then balance correction for the unknown unbalance distribution on any particular rotor may effectively be specified as two balance correction vectors, one in each of two preselected balance correction planes.
Of course, the terms "unbalance" and "balance" are used conventionally and as used herein are terms of degree. The degree of balance is selected for obtaining ideally no unbalance, or relatively little unbalance, in accordance with conventional practice.
A balance correction vector is defined by a specified mass or weight at a specified radial distance from the rotational centerline (stated, in combination, in units of gram-inches or the equivalent), at a specified angular (or "o'clock") position from some preselected reference point. The actual balance correction is then accomplished by removal of appropriate material at the proper radial and angular position at each of the two balance correction planes or, alternatively, addition of material at an angular location 180.degree. away from the specified location for material removal. Two-plane balance is the necessary and sufficient criterion for low-speed rigid-body balancing.
A rotor that runs to trans-critical or supercritical high speed (i.e. in the vicinity of or through and above a flexural critical speed which induces resonance) will require considerably more complex and refined balance, since the vibratory flexure of the rotor itself will further displace the unbalances from the rotor's rotational centerline and result in amplified excitation and vibratory response during operation at or near the critical speed(s). Each critical speed has associated with it a unique and different natural or critical mode shape, so that operation at or near each critical speed requires a unique and different two-plane balance correction. Accordingly, whereas low speed rotors can be fully balanced in two correction planes, rotors operating up to or through a single critical speed should be balanced in four planes; rotors operating up to or through two critical speeds should be balanced in six planes; and, in general, rotors operating up to or through N critical speeds should be balanced in 2(N+1) planes. This procedure is referred to as multi-plane or high-speed or modal balancing, and is conventional.
Fundamental to this requirement is the necessity of actually operating the rotor, in the course of its being balanced, at or near each of the N critical speeds (as well as at low speed) to collect the (N+1) sets of bearing reaction data. These data are then resolved mathematically into a specification of balance correction to be made at each of the 2(N+1) balance correction planes.
In practice, this balancing procedure is often done in situ, when the machine is first installed or is undergoing periodic overhaul or repair. If access to the balance correction planes is impractical in situ, then the rotor may be balanced in a component rig. But there are significant problems of feasibility and cost to be overcome in component test, particularly with high powered machinery. A basic need arises to accomplish the beneficial effects of high-speed balancing without actually having to operate the rotor at high speed in the course of the balancing operation.